Methacrylate or acrylate monomers are generally made by direct esterification of an alcohol or polyol with methacrylic acid or acrylic acid, or by transesterification of an alcohol or polyol with a lower ester of (meth)acrylic acid such as methyl methacrylate or ethyl acrylate. In such a process, a reactor is charged with the alcohol along with the desired (meth)acrylate reactant and a suitable catalyst. For instance, dodecanol is converted into dodecyl methacrylate with methacrylic acid and methane sulfonic acid. The reaction mixture is heated and the water produced as a byproduct in the process is removed as its azeotrope with a hydrocarbon solvent such as heptane. In the case of transesterification, an alcohol and methyl acrylate are heated in the presence of a tetraalkyl titanate, for instance, and the reaction is driven by removal of the byproduct, methanol. These reactions are widely practiced in industry using a large variety of specific catalysts, reaction conditions and equipment optimized for the particular monomers being manufactured. Ester interchange reactions are also well-known, particularly for the randomization of fats and oils. The use of alkali metal alkoxides is known for ester interchange reactions for fats and oils. See also U.S. Pat. No. 5,424,420, which discloses a method for preparing saccharide polyesters by transesterification using an alkali metal alkoxide.
Isobornyl methacrylate is a common monomer having utility because of the high glass transition temperature of its (co)polymers. It is conventionally prepared by acid catalyzed addition of methacrylic acid to camphene. This reaction proceeds, in the presence of an appropriate catalyst, rapidly and cleanly but to low conversion because it is an equilibrium reaction which cannot be driven by removal of a product. Thus, the acidic catalyst must be neutralized and the camphene and methacrylic acid separated from the product. Methacrylic acid is frequently lost in this separation and the camphene is a highly undesirable (because of its odor) impurity in the product. There is an obvious need for a new reaction scheme which avoids the complications of an equilibrium reaction which cannot be driven to high conversion and an offensive impurity. Another approach to preparing isobornyl methacrylate involves transesterification of isoborneol with methyl methacrylate; however, isoborneol is an extremely high melting "cakey" solid that is difficult to handle on an industrial scale.
A desirable methacrylate monomer is glycerol trimethacrylate, which is a trifunctional monomer having a low mass alcohol and a short distance between methacrylate moieties. Due to steric congestion surrounding the secondary alcohol and its ease of elimination, industrial syntheses of glycerol trimethacrylate have been impractical using conventional reaction sequences.
Typical polymerization inhibitors are phenolic compounds such as hydroquinone, 4-methoxyphenol, pyrogallol, or hindered phenolics such as butylatedhydroxytoluene and its derivatives. The oxygen in air serves as the initial radical chain stopper in these reactions since it reacts at diffusion controlled rates with carbon centered radicals which propagate chains in free radical polymerizations. The oxygen combines with the carbon centered radical to produce a much longer lived peroxy radical. The peroxy radical oxidizes a phenolic inhibitor and chain reactions are halted. Other classes of inhibitors include metal ions such as Cu(II), aromatic nitro compounds, nitrous oxide, nitroxyl compounds, aromatic amines, and aromatic heterocycles such as phenothiazine (PTZ). Some of these require air to function effectively and others do not. Some have great utility for some classes of monomers and others are not particularly useful for that class. The utility of some process inhibitors for a given class of monomers is even found to depend upon the reaction conditions being used for their preparation. For instance, phenolic inhibitors such as hydroquinone are particularly useful for preparation of (meth)acrylate esters by acid catalyzed direct esterification. Hydroquinone is a poor inhibitor for base catalyzed transesterifications because it rapidly oxidizes under basic conditions in the presence of air yielding dark reactions mixtures.
Polymerization inhibitors generally must be removed or diminished substantially in concentration before the product can be sold or polymerized by the end user. The high levels of inhibitor required to prevent polymerization during manufacture prevent effective polymerization. In the case of volatile monomers, such as allyl methacrylate, dimethylaminoethylmethacrylate, hexyl acrylate, or methyl acrylate, polymerization inhibitors can be removed via fractional distillation. Most inhibitors have significantly higher boiling points than distillable monomers and separation can be obtained. On the other hand, polymerization inhibitors cannot be removed from involatile monomers such as isodecyl acrylate, lauryl methacrylate, stearyl methacrylate, trimethylolpropane triacrylate and the like by distillation. Here, polymerization inhibitors must be removed via washing for example by removing a phenolic inhibitor with aqueous sodium hydroxide. In some cases it is undesirable to remove inhibitors via washing because of yield losses, miscibility, process complexity, or the desire for waste minimization. In other cases, low volatility of the monomer makes it difficult to remove low levels of a powerful inhibitor such as PTZ by distillation. Manufacturers of monomers are always interested in inhibitors which permit monomer process chemistry to proceed without polymerization while allowing easy separation of the product and inhibitor.
The present invention described hereinafter provides a novel reaction for the formation of alkyl (meth)acrylate monomers and fulfills the need in the field for the manufacture of such monomers using easily removed polymerization inhibitors.